1. Field of the Invention
The present invention relates to phased array antennas used in satellite communication systems. In particular, the invention relates to a method and a system for the digital beam forming at the transmit and/or receive side of a phased array antenna.
2. Background of the Invention
A phased array antenna is an antenna configuration useful to transmit and receive signals in a plurality of independent beams. The antenna aperture is subdivided into a plurality of sub-arrays in which each sub-array or patch consists in one or several radiating elements. The phase difference between the electromagnetic waves in the different beams determine the transmit or arrival direction of the beams. By applying appropriate phase shifts to the signals at each element, beams can be created and steered in any direction. In principle, separate phase shifts need to be performed at each patch for each beam.
Phase shifting can be performed by analog devices after low noise amplification on the receive side of a link and before high power amplification on the transmit side. It can also be performed by digital means using complex digital operations if the signal is converted into digital form.
Analog implementations are typically frequency-dependent and limited by the complexity of the interconnections and the precision of tuning (physical volume, losses, stability over age and temperature, manufacturing yield can become critical). Therefore, wideband implementations over a large field of view are difficult and a practical limit does also exist for the product number of beams times the number of patches. Digital implementations are limited by the power consumption, which is proportional to the signal bandwidth times the number of beams times the number of patches.
The concept of phased array antennas steered by beam forming has found several practical applications, namely in constellations of mobile satellite services operated in L or S band. These applications have been made possible by somehow favouable boundary conditions: beams are few or not steered in real time or anyhow carrying a very limited bandwidth which is well suited for digital implementation without major adaptations. Losses at L or S band are reasonably low for analog equipment. Each system operates in a reserved frequency band, so interference may not need tight control to conform to third party requirements. Furthermore, intra-system interference is mitigated by using signals which are orthogonal either in code or in time/frequency, thus somehow relaxing the need of side lobe control. The same concepts are not usable with future wideband systems, e.g. the upcoming K-band, on account of the difficulties that arise in that case.
The number of radiating elements per phased array increases by one order of magnitude to achieve the desired gain without creating unacceptable side lobes. The number of beams is also at the high end of current experience. Losses, mismatches, connections issues, tuning accuracy become more critical at such high frequency. Increased accuracy in beam steering is more and more required with the advent of non-geostationary systems.
All these aspects make analog beam forming extremely difficult, and especially as the new system generation will need tight shaping of side lobes and in general interference control. This growing need is due to the following factors:
1) inter-system coordination issues in an ever more packed frequency spectrum with several geostationary sharing the same frequency band,
2) intra-system tight interference control, originated by the push towards intensive frequency re-use to accomodate more traffic in the limited spectrum remaining available in K-band for generalised VSAT services (only 500 MHz out of 3 GHz).
Digital beam forming (DBFN) is therefore regarded as the natural solution to most of the problems referred to above. However, the complexity of the beam forming system is proportional to the number of sub-arrays or patches and to the number of beams and to the frequency bandwidth. The higher the complexity of the processing to be applied to the signal representative of the transmitted electromagnetic waves, the higher the power consumption required for the processing. The large number of patches and beams as well as the wideband characteristics of the future communication systems suggest that the power consumption achievable with conventional digital beam forming techniques will be prohibitive for satellite accomodation.
The digital beam forming (DBFN) schemes have been based so far on the principle of transforming the well known analog phase shift function into a directly equivalent digital operation, i.e. a complex multiplication. Various optimisations have been attempted on the digital algorithm to be used and on the numeric approximations, but always within the framework of the same basic principle of direct correspondence between the analog and digital schemes.
It is an object of the present invention to provide a novel and efficient digital beam forming method usable in steering a phased array antenna operating with a large number of wideband beams.
Another object of this invention is to provide a wideband digital beam forming system which needs a limited power consumption thereby to make it suitable for satellite implementation.
In accordance with the invention, the digital samples associated with each of the array elements arranged along a plurality of parallel lines are shifted by a distinct predetermined number of positions along each of said lines, and the digital samples of each line are added separately. Thereafter, each sum thus obtained is multiplied by a distinct phase coefficient. The signals thus obtained for each beam are all in phase. The lines of array elements that are electronically scanned can be oriented along any direction, and advantageously along one or a plurality of diagonals of the array and the electronic scanning of the array elements can be made separately along odd alternate diagonals and along even alternate diagonals.
The invention allows digital beam forming to be achieved using a number of multiplications that is considerably reduced as compared to the prior art systems. As a result, the signal processor needs a lower power consumption for the phase shift control. Such a reduced power consumption allows the method of the invention to be used in a great number of applications, and especially in applications where a limitation of the power consumption is of a primary importance, for instance in implementations on board a satellite.
The digital beam forming of this invention is particularly advantageous in applications which operate with a large number of beams. For an array antenna having a square lattice of 32xc3x9732 radiating elements generating 64 beams, each one with a 128 MHz bandwidth, the digital beam forming using the system of the invention only needs a power consumption in the order of 1 kW as against a power consumption in excess of 3 kW with a conventional digital beam forming scheme.
The digital beam forming system can be implemented using various arrangements of digital devices known per se.
The features and advantages of the invention will become more apparent by referring to the following detailed description in conjunction with the accompanying drawings.